1. Field of the Invention
This invention relates generally to a method of manufacturing a piston for internal combustion engines and more particularly to a method of manufacturing a piston for internal combustion engines which includes the step of forming a hollow chamber in a piston head for the passage of cooling oil therethrough.
2. Description of the Prior Art
In a conventional method of manufacturing a piston for internal combustion engines of this type, for example disclosed in U.S. Pat. No. 3,349,672, a piston assembly comprises a piston head and a collar. A hollow chamber is formed between the piston head and the collar. The seam between the piston head and the collar is welded by charged particle beam welding. However, in this method of manufacture, the following problems are encountered. For example, a relatively large amount of time is required for a welding process utilizing charged particle beams. In addition, the provision for a collar is necessary, thus resulting in an increase in the minimum number of parts in the manufacturing method. Furthermore, obtaining a preferred welding condition is difficult.
In a second conventional method of manufacturing a piston for internal combustion engines of this type, a core molding or casting system is used for forming a hollow chamber in a piston head in order to avoid the aforementioned problems. However, in this method the core is held by a graphite bar via a dead head portion of the piston head so as to prevent upward movement of the core during performance of the molding or casting method. Accordingly, the associated lack of space makes designing of the shape of piston head difficult, especially due to the positioning of the dead head portion with respect to the graphite bar. Furthermore, there is a requirement of withdrawing the graphite bar upon the molten metal added to the mold being caused to flow into the portion of the mold where the graphite bar is positioned. Therefore, an additional step in the method of manufacturing is necessary.